The present invention relates to a method and to an arrangement for measuring the cooling rate and thermal differential between the top and bottom of a printed circuit board (PCB). As defined, the invention is intended to facilitate control over the temperature differential which is encountered between the top and bottom of the PCB so as to prevent warpage thereof during the formation of component/module solder joints or fillets.
In the implementation of soldering procedures, for example, in reflow soldering ovens, which are employed in the soldering of high-mass PCBs, particularly when these PCBs are equipped with high-mass ceramic column grid array (CCGA) or ceramic ball grid array (CBGA) modules, there have been ascertained unique types of solder joint failures which are encountered in the formation of the module solder joints for producing electrical interconnections, and which are frequently referred to as solidification stress fractures. There are three types of failure mechanisms that can result;
(a) Solidification fractures, time-zero fails or reliability exposure (any solder fillet).
(b) Stretched or disturbed joints, reliability exposure (any solder fillet).
(c) Brittle cracked columns, time-zero fails or reliability exposure (Solder column modules only).
These modes of solder joint failure have been investigated in the technology, and shown to be highly dependent upon the cooling rate and thermal gradient extending through the thickness of the printed circuit board during corner solder joint or fillet solidification. An aspect which has been ascertained in cases of stress fracture solder joint failure has been the occurrence of a clean fracture which is produced between the intermetallic copper-tin (Cuxe2x80x94Sn) material on the circuit board solder joint pad and the solder material which is in the solder joint or fillet. The result of the foregoing can be either an almost immediate time-zero (instantaneous) open electrical interconnection, or a latent reliability fail which necessitates temperature cycling in order to become electrically open. The time-zero opens are characterized by gaps of up to 1 mil (0.001 inches), which may occur on only a single solder joint or fillet which is surrounded by a large number of so-called xe2x80x9cstretchedxe2x80x9d solder joints. The reliability failures may be separated through less than 100% of the soldered cross sectional area and do not evidence any measurable gap until failure during temperature cycling.
In essence, when a thermocoupled profile card is conducted or conveyed, such as on a belt or conveyor, through a reflow oven, for instance, an infrared (IR)/convection oven which is known to produce stress fracture interconnections, there has been indicated the presence of a sharp transient thermal differential spike between the top and bottom surfaces of the profile card, when thermocouple data is collected and plotted/analyzed as prescribed herein. This particular temperature differential spike is believed to be responsible for inducing printed circuit board warpage at a period in time when module solder joints or fillets especially at a corner and periphery of the module, are at the verge of solidification, thereby resulting in the type of failure mode previously described on nearby solder joints shortly after solidification.
For example, in cases where a pull test wire is soldered to a CCGA solder joint pad on a PCB, and shortly after solidification, it has been ascertained that the solder joint strength is extremely low; for example, approaching only a few grams. Moreover, from modeling studies, there has been indicated a variation in board warpage, which may be on the order of 1 mil. Thus, when these experimental observations are combined, this can readily result in an occurrence of stress fracture types of solder joint failure.
Another aspect of these particular solder joint failures which may not be readily apparent after assembly of the PCB components or modules, resides in the application of PCB deformation during solder joint solidification in effect, warping of the PCB during solder joint solidification, can readily weaken any resulting electrical interconnection solder joint or fillet of the components or modules.
The process causing solder joint failure and related influencing factors are essentially as follows:
1. The temperature differential between the top surface and bottom surface of the PCB (xcex94Tz=Ttopxe2x88x92Tbot) which is created extending through the thickness of the PCB (the Z-axis) during the cooling segment of the conveyor belt or rail driven oven reflow cycle. The temperature on the top surface of the printed circuit board can be significantly higher or lower than that on the bottom surface, depending on which surface cools more rapidly. One surface may cool faster than the other due to oven factors (forced air rate/volume/temperature, belt effects, etc.) and product attributes (component density and thermal mass). Thus, the magnitude of this Z-axis thermal differential is a function of
(a) PCB thickness inasmuch as thicker boards can withstand a larger temperature difference between the top or upper surface and the bottom surface.
(b) The mass, density and placement of components or modules on the top surface of the printed circuit board whereby the greater mass and density retains more heat on the top side or upper surface in comparison with the bottom side or surface.
(c) Cool-down rate wherein higher cooling rates exaggerate the instantaneous temperature differential between the top or upper and the bottom surfaces of the printed circuit board.
(d) The employment of direct impingement fans in order to cool the top and/or bottom PCB surfaces whereby one surface is cooled significantly faster than the other (instantaneously or during the entire cool down period) due to differences in
(1) Design and use of top versus bottom fans, staggered location of top/bottom fans, differences in airflow, and balanced use (some ovens have fans only on one side or the other).
(2) Design and use of oven belt or work board holder on which the PCB is placed, which can impede airflow more from one side than the other.
(3) Layout of components or modules on PCB top and/or bottom surface, which can also impede airflow locally across the PCB.
2. Differential thermal expansion between top and bottom board surfaces due to Z-axis temperature differential causes the printed circuit board to warp practically instantaneously, thereby imparting a load or stress on some of the component/module solder joints assembled thereon.
In the case of a positive xcex94Tz or change in the positive direction, the top side or upper surface of the printed circuit board expands at a greater rate than the bottom surface, causing the PCB to warp or bend concavely downwardly. In effect, below a module or component site, the PCB moves away from the module or component to the greatest extent at corner and edge solder joints.
3. The instantaneous warping of the printed circuit board with respect to the modules or components which are positioned thereon creates a displacement and a resultant load which can readily produce a disturbed or fractured solder joint or fillet, depending upon the timing and the temperature of the solder joints. This can occur in solder columns, balls, or fillets when the temperature range of the solder is from about (T1+10 degrees Celsius) greater than T2 greater than (T1xe2x88x9225 degrees Celsius) where T2 is defined as the instantaneous temperature of the solder joint and T1 is the solidification point of the solder joint upon cooling. The larger temperature range specified below T2 is intended to account for a phenomenon known as undercooling.
The result of the foregoing can be a time-zero electrical opening of the solder joints or fillets, and/or early life cycle reliability failures during thermal cycling.
Although considerations have been given in the technology towards improving the reliability of component or module solder joints on printed circuit boards, and particularly in monitoring and possibly regulating encountered temperature differentials as the printed circuits boards are conveyed through infrared ovens or solder reflow ovens on conveyor belts, transport rails or the like, these generally do not readily provide for simple remedial methods, measures, and arrangements which enable correction of the temperature differential spike problems which are encountered during soldering sequences and which cause PCB warping and resultant component or module solder joint failures or early life cycle reliability failures during thermal cycling.
Bast et al., U.S. Pat. No. 5,647,667 discloses an arrangement for the proof testing of ceramic parts. Utilized is an acceptance stress test in which a stress is generated by a temperature distribution on a part through thermal radiation. The stress is then characterized in order to validate the test.
Although the temperature distribution is characterized similarly to the process employed in the present invention as between various component regions, there is no teaching of a method and algorithm for the profiling of a printed circuit board transported through a reflow solder oven in order to obtain results which will be indicative of any temperature differential spikes tending to warp a PCB and adversely affect the integrity of solder joints and electrical connects.
Ume, U.S. Pat. No. 5,601,364 discloses a method and apparatus for measuring thermal warpage including a test setup and apparatus for producing shadow moire measurements on printed circuit boards over a given time interval and temperature profile. The temperature profile obtained thereby is a highly simplified simulation of an actual reflow oven, such as is obtained from typical oven profile cards. Although, the patent evaluates printed circuit board warpage for a desired temperature profile over a period of time, there is no disclosure of positioning thermocouples in a unique manner on opposite surfaces of a printed circuit board for detecting specific failure mechanisms, nor is the thermal differential (xcex94tz) between the top and bottom surfaces of the PCB ascertained.
Ito, U.S. Pat. No. 5,630,667 merely discloses a modeling method for predicting heat cracking through a three-dimensional polymer model constituted of photo-set resins having a particular thermal conductivity in order to identify hot spots which may be susceptible to heat cracking; for example, such as exhaust manifolds.
Marcantonio, U.S. Pat. No. 5,562,243 discloses a method and apparatus for reflow temperature settings, in which an artificial network provides for the reflow oven settings for acceptable soldering of printed circuit boards based on inputted thermal/physical features and feedback xe2x80x9clearningxe2x80x9d. There is no disclosure of the utilization of thermocouples on opposite PCB surfaces for detecting specific failure mechanisms when measuring temperature differential between the top and bottom surfaces of the printed circuit board.
Similarly, O""Rourke, et al., U.S. Pat. No. 4,180,199 also merely describes a mass soldering system for measuring temperature as a function of time through a wave soldering process, and does not direct itself to a method and arrangement pursuant to the present invention.
Accordingly, pursuant to the present invention, in order to be able to utilize a thermal profile card representative of a PCB, which will measure the cooling rate and the Z-thermal differential of the card, the localized temperature differential between the top and bottom surfaces of the card during a specific time interval is measured by a thermocouple pair while the module or component solder joints assembled thereon are solidifying. This is accomplished by providing the thermocouple pair at both the top and bottom surface of the thermal profile card at all critical points on the profile card, such as at the diagonal corner solder joints of the module or component assembled thereon, the center solder joint of the module or component, a leading edge of the card at three points, and a trailing edge of the card at three points and the center line of the card at three points, between the leading and trailing edges.
A recommended profile has to ensure that there is an absence of a significant change in temperature differential of the thermocoupled sensing pairs or thermocouples during the critical cooling period, as defined by thermocouple pairs located at or proximate module solder joints, when the thermal profile card is conveyed through the cooling segment of the reflow solder oven, whereby there will be substantially no encountered thermal profile card warpage while the solidifying solder joints are susceptible to fracture.
For example, examples of acceptable limits for xcex94Tz include, but are not limited to:
1. average magnitude of xcex94Tz of all pairs of thermocouples is in the range of about 0-12 degrees Celsius (xc2x0 C.).
2. No single magnitude of xcex94Tz is greater than about 10xc2x0 C.
3. No single change in the magnitude of xcex94Tz is greater than about 10xc2x0 C.
As previously described, it has become known that solidified solder joint or fillet strength is extremely weak at temperatures slightly below solidification temperature. Based on testing, a critical cooling period occurs when the temperature of the solder joints, for example solder columns, balls, or fillets, is from about (T1+10 degrees Celsius) greater than T2 greater than (T1xe2x88x9225 degrees Celsius). In view thereof, it is desired to eliminate any thermal differential spike which translates into a high Z-thermal differential and therefore profile card or PCB warpage, when the solder joints are within the range of about (T1+10 degrees Celsius) greater than T2 greater than (T1xe2x88x9225 degrees Celsius).
Accordingly, it is an object of the present invention to provide an arrangement for producing reliable component/module ball grid array (BGA) or column grid array (CGA) solder joint interconnections.
It is another object of the present invention to provide a method of utilizing a thermal profile card which will measure and simulate the cooling rate and the xcex94Tz between the top and bottom surfaces of a printed circuit board during the time interval while component or module corner solder joints are solidifying.
A more specific object of the present invention resides in providing an arrangement for measuring the cooling and Z-thermal differential through the utilization of thermocoupling the top surface and bottom surface of a printed circuit profile card so as to be able to compensate for temperature variations in a reflow solder oven and to thereby be able to avoid substantial temperature differential spikes.
According to one aspect of the invention there is provided a method for measuring the cooling rate and temperature differential between a top surface and a bottom surface of a printed circuit board, the method comprising the steps of providing the printed circuit board having a plurality of through-holes therein and including a module on the top surface thereon, the module having a plurality of lead free solder joints, at least one of the plurality of lead free solder joints being located relative to at least one of the plurality of through-holes, extending a thermocouple wire through at least one through-hole, and positioning a first thermocouple within the through-hole, within or proximate the at least one of the plurality of lead free solder joints, and connecting the thermocouple wire to the first thermocouple. The method includes positioning a second thermocouple on the bottom surface proximate the at least one through-hole and connecting the second thermocouple to the thermocouple wire to form a thermocouple pair with first thermocouple, reflowing the plurality of lead free solder joints, cooling the at least one lead free solder joint through a solidification temperature range, and measuring the temperature differential between the top and the bottom surfaces with the first and second thermocouples substantially through the solder joint solidification temperature range.
According to another aspect of the invention there is provided an arrangement for measuring the cooling rate and temperature differential between a top surface and a bottom surface of a printed circuit board comprising a module on the top surface of the printed circuit board, the printed circuit board including a plurality of through-holes therein, the module having a plurality of lead free solder joints, at least one of the lead free solder joints being located relative to at least one of the plurality of through-holes, a thermocouple wire positioned in the at least one through-hole, a first thermocouple positioned within the through-hole, within or proximate the at least one of the plurality of lead free solder joints and connected to the thermocouple wire, and a second thermocouple positioned on the bottom surface proximate the through-hole and connected to the thermocouple wire to form a thermocouple pair with the first thermocouple. The arrangement further includes a heater adapted for reflowing the plurality of lead free solder joints, a cooler adapted for cooling the at least one lead free solder joint through a solidification temperature range, and a device adapted for measuring the temperature differential between the top and the bottom surfaces with the first and second thermocouples substantially through the solidification temperature range.